Traditional CT scanners use energy-integrating detectors for acquiring energy integration X-ray data. An energy-integrating detector does not take advantage of the energy information in the X-ray beam. Even though the X-ray source emits X-rays in a broad spectrum, the detector is not able to differentiate between photons of different energy, but delivers an output signal proportional to the total energy of the photons registered during the readout interval. To obtain the spectral nature of the transmitted X-ray data, a photon-counting detector splits the X-ray beam into its component energies or spectrum bins and counts a number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as spectral CT. Spectral CT imaging provides material separation capabilities that can potentially enable new clinical applications. The spectral images are usually presented as material concentration images of basis materials or monoenergetic images. For example, spectral CT is used in discriminating tissues, differentiating between materials such as tissues containing calcium and iodine, or enhancing the detection of smaller vessels. Among other advantages, spectral CT is also expected to reduce beam-hardening artifacts and to increase accuracy in CT numbers independent of scanners.
Currently, most conventional designs acquire spectral information using either high- and low-energy X-ray sources or dual-detector-layer technologies. To improve the accuracy of material separation, photon counting detector technologies can be used to provide good energy resolution. Photon-counting energy-resolved direct-conversion semiconductor detectors for computed tomography (CT) allow exploitation of the spectral information of each incident photon. X-ray photons interacting with the semiconductor sensors can be converted directly to electron-hole pairs without any inefficient intermediate processes, ensuring the superior intrinsic energy resolution.
One of the key obstacles in performing a CT image reconstruction is the ability to make the noise uniform. In a conventional CT system, the noise is generally non-uniformly distributed in 3D space. In spectral CT, a monoenergetic image has an additional dimension along energy. Therefore, improving noise non-uniformity along the energy direction via X-ray source control is a challenge in spectral CT image reconstruction. For example, for the conventional kV switching technology, it is difficult to modulate mA levels between the two kVs, resulting in non-uniform noise in the energy direction.